This invention relates to d.c. motors fed by controlled converters in general and more particularly to a phase monitoring arrangement for a three phase network from which such motor is supplied which insures that operation that can be damaging to the motor or converter will not take place in the case of improper phase sequence or phase failure.
D.C. motors which are controlled by controlled converters in a three phase bridge circuit and which have their field circuits supplied by a rectifier bridge with a regulator providing inputs to a control circuit for generating firing pulses for the converter are known. In such devices, there is also provided a d.c. power supply to supply the necessary d.c. current to the regulator control circuit and the converter.
In drives of this nature, there is a danger to the converter, and particularly to their controlled switching elements, e.g., thyristors, which are sensitive to overloads if they are connected to the motor with the rotating field reversed because of an incorrect phase sequence. Similar problems occur in the event of a phase failure in the a.c. supply network. In particular, if a phase of the network fails there is a great danger, particularly if the failure occurs during the "standstill" control of the d.c. motor. The reason for this is that in the event of a failure of a phase of the network, the three phase bridge circuit of the converter operates like a single phase bridge circuit and the current regulator draws the required bridge current for the d.c. motor from the network through correspondingly fewer switching elements. This leads to larger a.c. currents in the inverter input and larger currents through the switching elements. Furthermore, the current ripple becomes larger and is accompanied by a degradation of the commutating properties resulting in brush arcing. Furthermore, with the smoothing of the actual current value input being the same, too low an actual current value is provided as an input to the current regulator. This causes the current limitation to be raised. Since the current transformer in the network feed line which is provided for measuring the actual current value is connected only to two phases, the actual current value provided as an input through the current regulator is reduced to less than one half if one of these phases fails. As a result, the current limitation assumes more than twice the value of the set magnitude and thus becomes practically ineffective as far as the switching elements are concerned. The greatest danger exists if a phase fails during standstill since the firing pulses are close to the inverter control limit of the three phase bridge circuit and a considerable residual voltage of .sqroot.2U sine 30.degree. is present on the d.c. side where a control angle of .alpha. = 150.degree. is set for the effective a.c. bridge. This residual voltage drives a large motor current, limited only by the ohmic resistance of the armature and therefor drives a large current through the switching elements.
In view of these problems, the need for a phase monitor which is capable of monitoring phase failures and phase sequence and of shuting down the d.c. motors where conditions which are not compatible with proper operation occur in order to protect the converter against overload becomes evident.